This invention relates to an improved process for manufacturing electrodes which are to be used in electrolytic cells, for example, a chlor-alkali electrolytic cell.
In a typical chlor-alkali electrolytic cell a saturated brine solution is electrolyzed by passing electric current through it in a specially designed cell having a diaphragm or a membrane between the cathode and the anode. Chlorine is produced at the anode while sodium hydroxide (NaOH) and hydrogen (H.sub.2) are formed at the cathode. Brine is fed continuously to the cells, while and Cl.sub.2, NaOH and H.sub.2 are continously withdrawn from the cells.
Currently, there are three types of electrolytic cells in commercial use for the production of chlor-alkali: (1) the mercury cell, (2) the diaphragm cell, and (3) the membrane cell. The operation of each of these cells is discussed in Volume 1 of the Third Edition of the Kirk-Othmer Encyclopedia of Chemical Technology page 799 et. seq.
The minimum voltage required to electrolyze an electrolyte into Cl.sub.2, NaOH and H.sub.2 may be calculated using the thermodynamic data. However, in commercial practice, the theoretical amount of voltage is not achievable and, higher voltages must be used to overcome the various resistances inherent in the various types of cells. To increase the efficiency of the operation of a diaphragm or a membrane cell one may attempt to reduce the overvoltages of the electrodes, to reduce the electrical resistance of the diaphragm or membrane, or reduce the electrical resistance of the brine being electrolyzed. The invention herein described results in an electrode particularly useful as a cathode in the electrolysis of brine; cathode overvoltage is substantially reduced, resulting in increased power efficiencies.
Because of the multi-million-ton quantity of alkali metal halides and water electrolyzed each year, even a reduction of as little as 0.05 volts in working voltage translates to meaningful economic savings, especially with today's constantly increasing power costs. Consequently, the industry has sought means to reduce this voltage requirement.
Throughout the development of chlor-alkali technology, various methods have been developed to reduce this cell voltage. Certain practitioners have concentrated on reducing cell voltage by modifying the physical design of the electrolytic cell, while others have concentrated their efforts on reducing the overvoltage at the anode or the cathode. The present invention concerns itself with an improved process to make a cathode that is characterized by a significantly lower overvoltage than cathodes that are generally used commercially.
It is well known that an electrode's overvoltage is a function of the current density and its composition (Reference: Physical Chemistry, 3rd ed.--W. J. Moore, Prentice Hall (1962), pp. 406-408), where the current density refers to the amperage applied per unit of true surface area of an electrode and composition refers to the chemical and physical makeup of the electrode. Therefore, a process that will increase an electrode's surface area will decrease its overvoltage at a given apparent current density. It is also desirable to use a composition of matter that is a good electrocatalyst; this further reduces the overvoltage.
It is well known in the art to use plasma or flame spraying to coat an electrode with an electroconductive metal. In U.S. Pat. No. 1,263,959 it was taught that anodes may be coated by spraying fine nickel particles onto an anode, wherein the particles are rendered molten and impacted on the iron substrate by means of a blast.
Cathodes, also, have been coated with electroconductive metals. In U.S. Pat. No. 3,992,278, cathodes were coated by plasma spraying or flame spraying an admixture of particulate cobalt and particulate zirconia. When these electrodes are used for the electrolysis of water or an aqueous alkali metal halide salt solution, they give prolonged lowering of hydrogen overvoltage.
Various metals and combinations of metals have been used to coat electrodes by plasma or flame spraying: U.S. Pat. No. 3,630,770 teaches the use of lanthanum boride; U.S. Pat. No. 3,649,355 teaches the use of tungsten or tungsten alloy; U.S. Pat. No. 3,788,968 teaches the use of titanium carbide or titanium nitride and at least one metal and/or metal oxide of the platinum group and a second oxide coating which is porous; U.S. Pat. No. 3,945,907 teaches the use of rhenium and; U.S. Pat. No. 3,974,058 teaches the use of cobalt as a coating with an overcoat of ruthenium.
It is, likewise, well known in the art to make porous electrodes by selective leaching. Coating an electrode with particulate nickel, then sintering the nickel as taught in U.S. Pat. Nos. 2,928,783 and 2,969,315; electrodepositing an alloy onto a substrate then leaching out one component of the alloy as taught in U.S. Pat. No. 3,272,788; pressing or cementing two or more components together or onto an electrode substrate and then selectively leaching out one or more of the coating components as illustrated by U.S. Pat. Nos. 3,316,159; 3,326,725; 3,427,204; 3,713,891 and 3,802,878.
It is also known in the art to combine the steps of making electrodes by plasma--or flame--spraying followed by leaching. It is also known to combine the steps of electroplating followed by leaching. Examples of known methods are illustrated in the following patents; U.S. Pat. No. 3,219,730 teaches coating a substrate with a multiple oxide film coating then removing the substrate by leaching, thus forming an electrode; U.S. Pat. No. 3,403,057 teaches flame or plasma spraying a Raney alloy onto a substrate followed by leaching aluminum out of the alloy, thus leaving a porous electrode; U.S. Pat. No. 3,492,720 teaches plasma spraying tungsten, titanium or alloys thereof along with aluminum, thorium and zirconium oxides onto a substrate. The substrate was subsequently removed, leaving a porous electrode.
U.S. Pat. No. 3,497,425 teaches preparing porous electrodes by coating the substrate with a relatively insoluble metal followed by a coating of a more easily dissolvable metal. The teaching requires heat treating to cause inter-diffusion of the two coats, while optimum conditions require separate heat treatments for each coat. The dissolvable metal is subsequently leached out, leaving a porous electrode. U.S. Pat. No. 3,618,136 teaches forming porous electrodes by coating a binary salt composition onto a substrate and leaching a soluble component from the system. The patent teaches that it is critical that the binary salt mixture is a eutectic composition and that optimum results are obtained when the same anions are used for both the active and the inactive salts, e.g. silver chloride--sodium chloride.
Netherlands Pat. Application No. 75-07550 teaches the preparation of porous cathodes by applying to a substrate a coating of at least one non-noble metal from the group of nickel, cobalt, chromium, manganese and iron, alloyed with a secondary, less noble, sacrificial metal followed by removal of at least a part of this sacrifical metal. Specifically, the sacrificial metal is chosen from the group of zinc, aluminum, magnesium and tin. The sacrificial metal is removed by leaching with a lye solution or an acid solution.
Japanese Pat. No. 31-6611 teaches forming a porous electrode by electroplating onto a substrate a nickel coating followed by a coating of zinc or some other soluble substance which is soluble in an alkaline solution. These coated electrodes are then either immersed in an alkaline solution or subjected to an electrochemical anodizing treatment to elute and remove zinc and other soluble substances, thus forming a porous electrode. Prior to immersion, a heat treatment of the coated electrode is required in some embodiments.
The present invention is a novel improvement over all of the known prior art of making electrodes with porous coatings by flame--or plasma--spraying followed by leaching, because of its incorporation of an inorganic compound as the pore former and because of the unique and unexpected benefits derived therefrom.
One benefit of the present invention is that it is significantly more economical than other known methods to produce electrodes with porous coatings by selective leaching. When the inorganic compound pore former of the present invention is used, such as sodium chloride, potassium chloride, calcium chloride, magnesium chloride, lithium chloride, etc., the cost of the pore former is much less than when metallic pore formers are used, such as aluminum, zinc, thorium, zirconium, manganese, iron and others taught in the prior art.
This invention does not require the use of an alloy as a coating material, as is required by several other known methods. Neither does the present invention require the use of a binary salt mixture, wherein the binary salt mixture is a eutectic composition nor is there need to treat the electrodes with heat after the coating has been applied but before leaching occurs, as is required by certain inventions of the prior art.
It is an object of the present invention to provide cathodes particularly well suited for use in electrolyzing aqueous alkali metal halide solutions in cells having diaphragm or membrane separators, which cathode has reduced hydrogen overvoltage, good life span, the ability to be produced from a variety of cathode substrates to desired configurations and which does not contain any contaminant which will be leachable into the electrolytic solution.
These and other benefits will be obvious to one skilled in the art to which this invention relates.